feat: Enhance chart title features with overlay, fill, and layout options

[debugdoctor]

Description

Add support for chart title customization: overlay mode, background fill, border style, and manual layout position. Introduces four new fields to the Chart struct while keeping Title []RichTextRun unchanged for full backward compatibility.

New fields added to Chart:

• TitleOverlay bool — whether the title overlaps the plot area
• TitleFill Fill — background fill for the title box
• TitleBorder ChartLine — border line style for the title box
• TitleLayout *ChartTitleLayout — manual position/size as fractions of chart area (0.0–1.0)

Internal changes:

• cTitle.Layout corrected from string to *cLayout for proper OOXML mapping
• Added cLayout and cManualLayout XML structs
• drawPlotAreaTitles refactored to accept a *chartTitle helper struct

Motivation and Context

Previously there was no way to control the chart title's visual style or exact position. Users who needed to overlay the title on the plot area, apply a background color, add a border, or pin the title to a specific region of the chart had no API to do so.

How Has This Been Tested

Unit tests added in chart_test.go covering:

• Chart title with manual layout (TitleLayout)
• Chart title with overlay + fill + border (TitleOverlay, TitleFill, TitleBorder)
• Chart title with border only
• Chart title with no overlay (default behavior)

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[xuri]

Thanks for your PR. I've left some comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (ae7a616) to head (495a1d5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2278   +/-   ##
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  Coverage   99.60%   99.60%           
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  Files          32       32           
  Lines       26654    26697   +43     
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+ Hits        26548    26591   +43     
  Misses         55       55           
  Partials       51       51           

Flag	Coverage Δ
unittests	99.60% <100.00%> (+<0.01%)	⬆️

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[debugdoctor]

Great

[xuri]

The screenshot below shows the line formatting settings panels for both shape outlines and chart title lines. As you can see, properties such as line type and width are consistent across both, which allows us to reuse the same data structure for line formatting.

Based on your branch, I have introduced some breaking changes, including the adoption of LintOptions for both shapes and charts.